The present invention relates to a mode switch for a magnetic recording and reproducing apparatus such as a VTR which detects an operating position of a loading mechanism for loading or unloading tapes as well as a control method for the mode switch.
FIGS. 8 and 9 show a mode switch 9 used for conventional magnetic recording and reproducing apparatuses.
The mode switch 9 comprises a bearing 1a formed in the center of a housing 1 formed of an insulator, and a common pattern 2 and a detection pattern 3 each formed concentrically in an inner bottom portion of the housing 1 on an end surface thereof and composed of a conductive material such as copper.
Reference numeral 4 denotes a rotor formed of an insulator and having a boss portion 4a which is formed in the center thereof and which is rotatably engaged with the bearing 1a of the housing 1. The rotor 4 has a brush 5 formed on an end surface thereof and composed of a conductive material, the brush 5 rotating integrally with the rotor 4. The brush 5 maintains contact with the common pattern 2 and the detection pattern 3 and is rotated while being arranged in a substantially radial direction in a line.
As shown in FIG. 9, the detection pattern 3 is divided into four position detecting sections A to D having terminals a1, b1, c1, and d1, respectively, extended therefrom. These detection positions are electrically separated and are not electrically connected together.
FIG. 10 shows the mechanism of a magnetic recording and reproducing apparatus using the mode switch 9. Reference numeral 6 denotes a motor operated by means of outputs from a system control circuit 20.
Rotational outputs from the motor 6 are joined to a loading mechanism 21 via a gear 7 and a reduction gear 8. The loading mechanism 21 loads and unloads a tape. While this mechanism is operative, the mode switch 9, having the rotor 4 joined to the reduction gear 8, rotates and the common pattern 2 on an inner circumferential side and each detection pattern 3 on an outer circumferential side are short-circuited by brush 5 in conjunction with the rotation to enter a conductive or open state.
A common terminal COM of the mode switch 9 is connected to an L (Low) level, while the terminals a1, b1, c1, and d1 of the mode switch 9 are pulled up to an H (High) level, with these levels input to the system control circuit 20 so that the rotation of the motor 6 can be controlled by means of a microcomputer (not shown) to achieve appropriate operations.
FIG. 11 shows a signal which is input to the system control circuit 20 if the mode switch 9 is rotated, and when the brush 5 is rotated once counterclockwise from between the detection positions A and D shown in FIG. 9.
For example, at the detection position A, the brush 5 is in contact with the common pattern 2 and with the detection pattern connected to the terminal a1. In FIG. 10, a switch for the terminal al of the mode switch 9 becomes conductive and is pulled up, so that an input from the terminal a1 to the system control circuit 20 has the L level, while the other terminals are in the open state and thus have the H level.
Likewise, at the detection position B, the terminal b1 has the L level, while the other terminals have the H level. At the detection position C, the terminal c1 has the L level, while the other terminals have the H level. At the detection position D, the terminal d1 has the L level, while the other terminals have the H level.
Since the common pattern 2 is electrically connected to no detection pattern at passed positions other than the detection position, the outputs from the terminals a1 to d1 all have the H level. The logic of the signal input to the system control circuit 20 varies depending on the detection position A to D, so that checking the logic of the input from the mode switch 9 enables the loading mechanism to be detected at the position A to D.
FIGS. 12(a), (b), and (c) show how a tape is loaded.
Reference numerals 10, 11, 12, and 13 denote a cassette, a magnetic tape, a drawing post, and a cylinder. FIG. 12(a) shows an unloading state where the brush 5 of the mode switch 9 is at the detection position A. FIG. 12(b) shows a half loading state where the brush 5 of the mode switch 9 is at the detection position B. FIG. 12(c) shows a loading completed state where the brush 5 of the mode switch 9 is at the detection position D. The position of the brush 5 of the mode switch 9 moves in connection with the loading mechanism 21 as shown in FIG. 10, so that each loading position corresponds to the detection position.
Thus detecting the output from the mode switch 9 enables the position of the loading mechanism 21 to be detected, and controlling the motor 6 enables the tape loading operation to be controlled.
Since the detection positions A to D of the mode switch 9 shown in FIG. 9 each correspond to one of the loading positions, checking the output of each terminal enables its absolute position to be determined. This type is hereafter referred to as an xe2x80x9cabsolute-position detection typexe2x80x9d.
FIG. 13 is another type (relative-logic-based detection type) of mode switch 9.
The detection patterns 3 connected to the terminals a2, b2, and c2 are sequentially and concentrically formed outward from a central portion of the mode switch, with the common patterns 2 concentrically formed in an outermost periphery of the mode switch and each connected to a terminal COM. Portions J of the same concentric track which are shown by broken lines are i electrically connected together but the surfaces thereof are molded of an insulator such as a resin.
FIG. 14 shows input signals to the system control circuit 20 shown in FIG. 10, which are obtained if the mode switch 9 is rotated. The signals shown in FIG. 14 are obtained when the brush 5 shown in FIG. 13 is rotated once counterclockwise from between the detection positions A and D.
In the case of the absolute-position detection type mode switch 9 shown in FIG. 9, timings for the detection positions depend on the pattern length of each detection pattern 3 at the detection position A to D. In the case of the mode switch 9 shown in FIG. 13, however, the common pattern 2 is divided into the positions A to D and the timings for the detection positions do not depend on the length of the detection pattern 3 but generally on the length of each position A to D of the common pattern 2. Since the closer to the outermost concentric circle the position is, the longer the circumferential length per angle is and the higher the angular precision obtained is, the common pattern 2 is disposed peripherally.
Also in the mode switch 9 shown in FIG. 13, the logic of the output of the terminal a2, b2, or c2 varies depending on the detection position A to D, so that determining the logic enables the loading position to be detected. In this type of mode switch, the detection patterns 3 connected to the terminals a2, b2, and c2 do not correspond to the different loading positions, but determining the output logic of the terminals a2, b2, and c2 enables the loading position to be determined. This type is hereafter referred to as a xe2x80x9crelative-logic-based detection typexe2x80x9d.
To realize many functions of a magnetic recording and reproducing apparatus such as a VTR, inputs from many switches or sensors are connected to the microcomputer in the system control circuit 20, so that a mode switch is required which obtains accurate outputs using as few inputs to the system control unit 20 as possible.
In the xe2x80x9cabsolute-position detection typexe2x80x9d mode switch 9, since each terminal corresponds to one of the loading positions, if chattering or noise occurs in the brush, the terminal may have the same logic as any of the passed positions but never has that as the other detection positions. This prevents an incorrect detection position from being detected as the loading position.
A known control method that is used if noise occurs in the xe2x80x9cabsolute-position detection typexe2x80x9d mode switch 9 is described in Japanese Patent Laid-Open No. 1-67748. With this method, a drive mechanism is stopped simultaneously with a change in the output level of the mode switch, and a fixed period of time later, the mode switch is checked to restart the drive mechanism if the output has changed. This operation is repeated to stop the mode switch at a regular position.
The xe2x80x9cabsolute-position detection typexe2x80x9d mode switch requires as many detection positions 3 and terminals as the detection positions. Since the number of terminals increases consistently with the number of detection positions, an increase disadvantageously occurs in the number of wires for connecting the mode switch 9 and the system control circuit 20 together and in the number of input ports to the system control circuit 20.
On the other hand, the xe2x80x9crelative-logic-based detection typexe2x80x9d requires fewer terminals than the xe2x80x9cabsolute-position detection typexe2x80x9d, but an incorrect detection position is detected as the loading position if noise occurs in the output. If, for example, at the detection position B, chattering or the like in the brush 5 causes a high noise to change the output of the terminal c3 to H level, as shown in FIG. 15, the terminal a2, b2, and c2, which should actually have the L, H, and L levels, respectively, at this position, have the L, H, and H levels, respectively, constituting the same logic as that established at the detection position D.
FIG. 17 shows an example of a conventional mode switch control method wherein the process starts from the detection position A and stops at the detection position D. A general method comprises periodically reading the output of the mode switch 9 a number of times to establish the position detected with the same logic established.
FIG. 16 is a view useful in explaining timings with which the output of the mode switch is read, and FIG. 15 shows details of noise occurring at the terminal c2 of the detection position B.
In FIG. 16,  less than 1 greater than to  less than 5 greater than show timings with which the output of the mode switch is checked using fixed cycles. The line shown by the alternate long and short dash line denotes a threshold above which the level is determined to be H and below which it is determined to be L. For example, the output is continuously checked three times, and if the same logic is detected, that logic is established. The broken line denotes noise. At  less than 1 greater than , the output is at the L level. At  less than 2 greater than , noise lower than the threshold level occurs and the output is at the L level. At  less than 3 greater than to  less than 5 greater than , the H level is read due to noise. Since the H level is continuously read three times, the level is determined to be H upon the reading at  less than 5 greater than .
With the reading based on this method, in case of FIG. 15, if the level of noise exceeds the threshold and its width is longer than a timing for establishing the logic, the logic is mistakenly determined, causing the mode switch to be stopped at the position D, which should actually be the position B. As a result, the apparatus may malfunction.
It is an object of the present invention to provide a mode switch that obtains reliable output signals with a small number of terminals as well as a control method therefor.
To attain this object, the present invention provides a relative-logic-based detection type mode switch wherein if an output signal different from that corresponding to the turn of a predicted detection position is obtained, that output signal is neglected, whereas only if a logic signal for the predicted detection position is obtained, that logic signal is established.
This configuration reduces the possibility of misdetection even if noise occurs during the process to provide the same logic at an incorrect detection position. The possibility of misdetection is further reduced by checking whether a logic signal for a passed position before or after the detection position is obtained.
In addition, an abnormal-position-detection position that allows more common and detection patterns to be electrically connected together than in a normal loading and unloading operating range is provided outside the same range so that upon detecting an abnormal-position-detection pattern, a loading or an unloading operation is stopped or the loading operation is switched to the unloading operation and vice versa. Then, even if misdetection causes the detection position to be passed, detecting the abnormal-position-detection position enables appropriate control to prevent malfunctions. Different combinations of common and detection patterns are electrically connected together at different positions, and the number of detection patterns that are electrically connected to the common pattern is fixed at a constant number of two or larger. This reduces the possibility of mistakenly detecting an incorrect detection position as the loading position even if noise such as chattering occurs.
As described above, according to the present invention, a relative-position-based detection type mode switch with fewer terminals gives an advantageous effect of providing a reliable output that is unlikely to cause an incorrect detection position to be detected as the loading position. It also has an effect of facilitating creation of a control program for the mode switch due to considerations for the order of the logic of outputs from the mode switch.